1. Field of the Invention
The present invention relates to a distance-measuring apparatus for measuring the distance to an object and, more particularly, to an active distance-measuring apparatus used in cameras and other equipment.
2. Related Background Art
The active distance-measuring apparatus used in the cameras etc. is arranged to project light from an infrared-emitting diode (hereinafter referred to as xe2x80x9cIREDxe2x80x9d) toward the measured object, to receive reflected light of the projected light by a position sensing device (hereinafter referred to as xe2x80x9cPSDxe2x80x9d), to process a signal from this PSD by a signal processing circuit and an arithmetic circuit to output distance information, and to determine the distance to the measured object by a CPU. Since the distance measurement with only one projection of light can cause an error, it can be considered that a plurality of light projection operations are carried out to gain a plurality of distance information pieces and that the plurality of distance information pieces are integrated by an integrating circuit to be averaged. In this case, in the integrating circuit a fixed reference voltage is applied to an integrating capacitor and the integrating capacitor is gradually discharged according to each distance information. After completion of the predetermined number of light projection operations, the integrating capacitor is then charged by constant current to measure the time to the reference voltage, and the distance to the measured object can be computed based on the time.
In the distance-measuring apparatus described above, however, an error can arise in the result of the distance measurement, too. For example, when the reference voltage is applied to the integrating capacitor, the voltage between the terminals of the integrating capacitor becomes lower than the reference voltage because of dielectric polarization. For this reason, the charging time of the integrating capacitor is not related correctly to the distance to the measured object, so that the apparatus may fail to gain the accurate measurement result.
In order to avoid this problem, it can be contemplated that a voltage higher than the reference voltage is preliminarily applied to the integrating capacitor and then the voltage applied is changed to the reference voltage after a lapse of a certain time. In this case, however, the time for the measurement becomes longer by the time of the precharge at the voltage higher than the reference voltage. Particularly, where charge and discharge steps of the integrating capacitor are carried out several times to compute a plurality of distance measurement results and where the distance to the measured object is determined as an average of the results, the time of precharges is summed up to become longer, thus making great time parallax.
Therefore, the present invention has been accomplished in order to solve such a technical problem and an object of the present invention is to provide a distance-measuring apparatus that can obtain an accurate distance measurement result while decreasing the time parallax.
For accomplishing the above object, a distance-measuring apparatus according to the present invention is a distance-measuring apparatus comprising: light projecting means for projecting light toward a measured object; light receiving means for receiving reflected light of the light projected toward the measured object, at a photoreceptive position on a position sensing device according to a distance to the measured object and outputting a signal according to the photoreceptive position; arithmetic means for carrying out an arithmetic operation based on the output signal outputted from the light receiving means and outputting a distance signal according to the distance to the measured object; integrating means comprising an integrating capacitor, the integrating means carrying out a first integral in which the signal outputted from the arithmetic means is integrated by discharging/charging the integrating capacitor according to the signal outputted from the arithmetic means and thereafter carrying out a second integral by charging/discharging the integrating capacitor at a constant current, the integrating means comparing a voltage of the integrating capacitor with a reference voltage during the second integral and outputting a comparison result signal according to a result of the comparison; detecting means for detecting the distance to the measured object, based on the signal outputted from the integrating means; and charging means for carrying out precharge by preliminarily applying a constant voltage to the integrating capacitor, prior to execution of the first integral by the integrating means; wherein a plurality of distance measurement operations are carried out by repeatedly carrying out the projection of light by the light projecting means, the reception of light by the light receiving means, the arithmetic operation by the arithmetic means, and the integral operation by the integrating means and the detecting means detects the distance to the measured object, based on results of the respective distance measurement operations; and wherein the precharge by the charging means is carried out prior to the first distance measurement operation and the precharge is not carried out prior to the second and subsequent distance measurement operations.
The distance-measuring apparatus according to the present invention is also characterized in that the arithmetic means comprises a stationary-light-removing capacitor, the arithmetic means makes the stationary-light-removing capacitor store charge corresponding to an output level of the output signal of the light receiving means while the light projecting means projects no light, and the arithmetic means outputs the distance signal according to the distance to the measured object by removing an output component due to stationary light except for the light projected from the light projecting means out of the output signal of the light receiving means in accordance with a state of the storage of the stationary-light-removing capacitor while the light projecting means projects light, wherein the charging means carries out precharge by preliminarily applying a constant voltage to the stationary-light-removing capacitor, prior to execution of the first integral by the integrating means, and wherein the precharge of the integrating capacitor and the stationary-light-removing capacitor is carried out by the charging means prior to the first distance measurement operation and wherein the precharge of the integrating capacitor and the stationary-light-removing capacitor is not carried out prior to the second and later distance measurement operations.
According to the present invention, the apparatus can obtain the accurate distance, because the apparatus is arranged to carry out the plural distance measurement operations and compute the distance to the measured object, based on the results of the plural distance measurement operations. In addition, the apparatus can decrease the distance measurement time and decrease the time parallax between the start of shutter release operation and the end of exposure, because the apparatus is arranged not to carry out the precharge of the integrating capacitor or the stationary-light-removing capacitor in the second and subsequent distance measurement operations.
Another distance-measuring apparatus according to the present invention is a distance-measuring apparatus comprising: light projecting means for projecting light toward a measured object; light receiving means for receiving reflected light of the light projected toward the measured object, at a photoreceptive position on a position sensing device according to a distance to the measured object and outputting a signal according to the photoreceptive position; arithmetic means for carrying out an arithmetic operation based on the output signal outputted from the light receiving means and outputting a distance signal according to the distance to the measured object; integrating means comprising an integrating capacitor, the integrating means carrying out a first integral in which the signal outputted from the arithmetic means is integrated by discharging/charging the integrating capacitor according to the signal outputted from the arithmetic means and thereafter carrying out a second integral by charging/discharging the integrating capacitor at a constant current, the integrating means comparing a voltage of the integrating capacitor with a reference voltage during the second integral and outputting a comparison result signal according to a result of the comparison; detecting means for detecting the distance to the measured object, based on the signal outputted from the integrating means; and charging means for carrying out precharge by preliminarily applying a constant voltage to the integrating capacitor, prior to execution of the first integral by the integrating means; wherein a plurality of distance measurement operations are carried out by repeatedly carrying out the projection of light by the light projecting means, the reception of light by the light receiving means, the arithmetic operation by the arithmetic means, and the integral operation by the integrating means and the detecting means detects the distance to the measured object, based on results of the respective distance measurement operations; and wherein the precharge by the charging means is carried out prior to the first distance measurement operation and the precharge is carried out in a shorter time prior to the second and subsequent distance measurement operations than a time of the precharge prior to the first distance measurement operation.
The distance-measuring apparatus according to the present invention is also characterized in that the arithmetic means comprises a stationary-light-removing capacitor, the arithmetic means makes the stationary-light-removing capacitor store charge corresponding to an output level of the output signal of the light receiving means while the light projecting means projects no light, and the arithmetic means outputs the distance signal according to the distance to the measured object by removing an output component due to stationary light except for the light projected from the light projecting means out of the output signal of the light receiving means in accordance with a state of the storage of the stationary-light-removing capacitor while the light projecting means projects light, wherein the charging means carries out precharge by preliminarily applying a constant voltage to the stationary-light-removing capacitor, prior to execution of the first integral by the integrating means, and wherein the precharge of the integrating capacitor and the stationary-light-removing capacitor is carried out by the charging means prior to the first distance measurement operation and wherein the precharge of the integrating capacitor and the stationary-light-removing capacitor is carried out in a shorter time prior to the second and subsequent distance measurement operations than a time of the precharge prior to the first distance measurement operation.
According to the present invention, the apparatus can obtain the accurate distance, because the apparatus is arranged to carry out the plural distance measurement operations and compute the distance to the measured object, based on the results of the plural distance measurement operations. In addition, the apparatus can decrease the distance measurement time and decrease the time parallax between the start of shutter release operation and the end of exposure, because the apparatus is arranged to shorten the period of the precharge of the integrating capacitor or the stationary-light-removing capacitor in the second and subsequent distance measurement operations.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.